Full-duplex radio frequency echo cancellation

ABSTRACT

A system comprising a transmitter element creating an interrogation signal and transmitting the interrogation signal and a receiver element receiving a reflection signal of the interrogation signal and combining the reflection signal and a feedback signal to cancel at least a portion of radio frequency echo signals in the reflection signal.

BACKGROUND

Radio frequency identification (“RFID”) systems are used in a plethoraof commercial contexts requiring a unique identification system forlarge numbers of items. Such contexts include everything from departmentstore inventory and check-out systems to the tracking of militarysupplies to and from the front lines. Similar in utility to bar codetechnology, RFID systems are often preferred due to their increasedrange, lack of a line of sight requirement between a tag and its readerand the high multi-tag throughput of RFID readers (i.e., RFID readersmay read many tags in their large field of view at very high transportspeeds).

A problem that arises is that optimal performance of RFID systems isoften hampered by the reflection and coupling which inevitably occur inRF transceivers, in which a significant portion of the transmittedinterrogation signal is reflected by the antenna and objects in theenvironment into the receiving portion of the transceiver. Theseproblems are quantified in a measure called the voltage standing waveratio (“VSWR”), measured as the non-transmitted (i.e. coupled orreflected from the antenna or non-RFID objects in the environment) powerover the total transmitted power of the transceiver. A high VSWRinterferes with efficient transceiver performance and may even result ina “blinding” or complete saturation of the receiver. Transceiversdesigned to minimize VSWR are often unacceptable because of their highcost in terms of size and power, especially in the context of mobiledevices.

SUMMARY OF THE INVENTION

A system comprising a transmitter element creating an interrogationsignal and transmitting the interrogation signal and a receiver elementreceiving a reflection signal of the interrogation signal and combiningthe reflection signal and a feedback signal to cancel at least a portionof radio frequency echo signals in the reflection signal.

A method, comprising the steps of receiving a reflection signal,deriving a feedback signal from the reflection signal by isolating anerror component of the reflection signal and combining the reflectionsignal and the feedback signal to cancel at least a portion of radiofrequency echo signals in the reflection signal.

Furthermore, a method comprising the steps of demodulating a reflectionsignal into an in-phase signal and a quadrature signal, filtering thein-phase signal to isolate an in-phase error signal, filtering thequadrature signal to isolate a quadrature error signal, modulating thein-phase error signal and the quadrature error signal to create afeedback signal and combining the reflection signal and the feedbacksignal to cancel at least a portion of radio frequency echo signals inthe reflection signal.

In addition, a system, comprising a demodulator to demodulate areflection signal into an in-phase signal and a quadrature signal, afirst filter to isolate an in-phase error signal from the in-phasesignal, a second filter to isolate a quadrature error signal from thequadrature signal, a modulator to modulate the in-phase error signal andthe quadrature error signal to create a feedback signal and a combinerelement to combine the reflection signal and the feedback signal tocancel at least a portion of radio frequency echo signals in thereflection signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the basic components of a conventional RFID system andtheir locations with respect to one another within such a system.

FIG. 2 shows an exemplary embodiment according to the present inventionof a feedback loop inserted into the transceiver component of an RFIDsystem enabling VSWR error signal cancellation.

FIG. 3 shows an alternative embodiment of the present inventionsubstituting a 4-way directional coupler for the circulator element andRF splitter shown in FIG. 1.

FIG. 4 shows an alternative embodiment of the present invention withnon-linear error rejection used for RF echo cancellation.

FIG. 5 shows an alternative embodiment of the present inventionemploying digital signal filtering in the feedback loop rather thananalog filtering.

FIG. 6 shows the preferred embodiment of the present invention.

FIG. 7 shows the sub-system interfaces through which the transceiverdescribed using the present invention may interact with other componentsof an RFID system.

DETAILED DESCRIPTION

FIG. 1 illustrates the basic components of an RFID system 1, in which RF“tags” located on objects in the environment may reflect radio wavesoriginating from a transceiver 10 in a pre-determined bit pattern anddata rate via the principle of backscatter radiation. These reflectionsmay be received by the transceiver 10, stripped of their carrier signaland converted into in-phase (“I”) and quadrature (“Q”) components. Thesecomponents may then be independently digitized and sent to a base-bandprocessor 20 for bit decoding. This decoded information may then be senton to a reader control 30 which may perform such processes as errorcorrection, command interpretation, and management of access to the RFchannel. A host interface 40 performs filtering operations andtranslation of the results of the reader control 30 into a formatintelligible to the host, and likewise translates host requests into aformat intelligible to the reader control 30.

FIG. 2 shows an exemplary embodiment of a transceiver component of anRFID system according to the present invention. The exemplary tranceivercomponent includes a feedback loop which serves to minimize the effectsof reflection and coupling on the incoming signal, and resulting in alower transceiver VSWR.

The transmitter portion of an RFID transceiver may create aninterrogation signal for transmission by using a modulator 105 and avariable gain amplifier (“VGA”) 110 to modulate a carrier signal. Use ofthe VGA 110 may result in an amplitude modulated (AM) carrier wave. Thismodulated carrier wave may then be sent to a power amplifier 115 andband-pass filter 120. This amplified and filtered modulated carrier wavemay then be sent to a circulator or coupler element 125 for transmissionto the antenna 130.

This transmitted interrogation signal may then reflect off of an RF tagwhich has been attached to or associated with a piece of equipment orother commodity. These reflections, which carry information to identifythe tag, may be received by the antenna 130. In an ideal RFIDtranceiver, these received reflections constitute the whole of thesignal received by the antenna 130. However, in deployed RFID systems,the received signal also contains an error component comprised ofinterrogation signal energy which has been coupled from the transmitter,reflected from the antenna 130, and reflected from objects in theenvironment other than the RF tag.

The incoming signal may arrive at the antenna 130 containing bothvaluable information from an RF tag and an error signal. In theexemplary embodiment of the present invention, this composite signal maybe sent through a circulator 125 which may route the incoming signalinto one input of an RF combiner 140. The combiner 140 may add thisincoming signal to the output of the feedback circuit discussed below,and may feed the sum of these two signals into a band-pass filter 145.The band-pass filter 145 removes signal components outside of thefrequency range of the modulated data signal of interest.

The signal may then be amplified by an automatic gain control (“AGC”)150. This amplified signal may then be carrier-demodulated in quadratureusing a demodulator 155. Both of the resulting demodulated signals (thein-phase signal Irx and the quadrature signal Qrx) may then be split.Two separate branches may take the in-phase and quadrature signalsthrough band-pass filters 180I and 180Q before continuing towards thetransceiver output for further processing by the base-band decoder 20.

Each of these branches includes a second path as input for a feedbackloop. The feedback loop achieves echo cancellation in the transceiver byisolating the noise (error) component of the incoming signal usinglow-pass filters 160I and 160Q, subjecting this signal to a phaseinversion, and then combining it with the incoming signal using anotherinput of the RF combiner 140. The required phase inversion may beaccomplished by modulating the physical path length of the return loop.For example, the path length may be controlled by either controlling themicrowave traces on the circuit board at the design phase, or by addinga variable delay element for adaptive control. The feedback loop may bedesigned to converge with the incoming signal within the impulseresponse time of the low-pass filter, which is usually within a fewcycles of the carrier signal.

After beginning the feedback loop, both the in-phase signal Irx and thequadrature signal Qrx may first be passed through low-pass filters 160Iand 160Q. These low-pass filters may isolate the undesirable echo signalsince the majority of the base band error signal is of a lower frequencythan the signal of interest. In this example, the error signal is of alower frequency and therefore low pass filters are used. However, theremay be other implementations where the error signal is in a definedrange of frequencies and a band-pass filter may be used or where theerror signal is a higher frequency signal and a high pass filter isused. The outputs of these low-pass filters 160I and 160Q may then bemodulated using modulator 165. The two signals may then be combinedusing a summing element 170. The resulting signal may then be passedthrough feedback amplifier 175 and the amplified signal may be fed intoanother input of RF combiner 140. This closes the feedback loop. Thefeedback signal may combine with the incoming signal in a manner whichcancels out the noise component of the incoming signal, leaving only themodulated data reflected from the RF tag.

FIG. 3 shows an alternative exemplary embodiment of the presentinvention. This embodiment may achieve similar results using a lowernumber of overall elements by replacing the circulator element 125 andthe RF splitter 140 depicted in FIG. 2 with a 4-way directional coupler205. The interrogation signal is synthesized in this embodiment in thesame manner described for FIG. 2 above. The incoming signal from theantenna 130 may be sent into one port of the coupler 205 which in turnmay pass this incoming signal to output port R of the coupler 205. Thesignal path from that point is the same as that described in FIG. 2above, beginning with the receiver band-pass filter 145.

The exemplary embodiment of the present invention shown in FIG. 3 may bemore cost-effective than that shown in FIG. 2, either by reducing thenumber of components in the transceiver or by substituting lessexpensive yet equally effective components for more expensive ones.However, this arrangement may introduce other problems such asnon-linearity and amplification of harmonics. The non-linearity may becontrolled by recording it digitally and then adding a correction factorinto the feedback loop. Amplified harmonics may be controlled by addinga low-pass filter (not shown) to the output of the feedback amplifier175.

FIG. 4 shows a second alternative exemplary embodiment of the presentinvention with a sample and hold circuit 305 inserted into the feedbackloop. Both the outgoing and incoming signal paths are the same in thisembodiment as those described in FIGS. 2 and 3 above; in addition,however, the sample and hold circuit 305 (shown as sample and holdcomponents 305I and 305Q) may be inserted in the feedback loop inbetween the low-pass filters 160I and 160Q. The sample and hold circuit305 may cancel the static reflection components of the received signalby activating its hold mode when the transceiver is receiving abackscatter signal. While the sample and hold circuit 305 may not cancelnoise components caused by slow movements in the environment, theeffects of these movements may be minimized due to the long duration ofsuch reflection changes relative to the hold periods of the circuit. Thesample and hold circuit 305 may also result in an overall reduction innoise caused by coupling between the In-phase (Irx) and Quadrature (Qrx)components of the received signal.

FIG. 5 shows a third alternative exemplary embodiment of the presentinvention using a base-band digital radio 410 to accomplish thefiltering portion of the feedback loop digitally. Here, the demodulatedIn-phase (Irx) and Quadrature (Qrx) components of the received signalmay be converted into digital signals using analog-to-digital converters415I and 415Q. The output signal path from the transceiver 10 to thebase band processor 20 is the same as in the previous embodiments,except that the low-pass filters 420I and 420Q are implemented asdigital components in the base band digital radio 410.

In the feedback portion of the signal path, these digital signals maythen be filtered using digital low-pass filters 420I and 420Q containedin the base-band digital radio 410. The output of these filters may thenbe converted back into analog signals using digital-to-analog converters425I and 425Q. These converters may inherently perform the echocancellation performed by the sample-and-hold circuit 305 in FIG. 4.Thus, the inclusion of the base band digital radio 410 in this exemplaryembodiment obviates the need for a sample-and-hold circuit of the kindpresented in FIG. 4.

FIG. 6 shows a fourth exemplary embodiment of the present invention.This exemplary embodiment creates the outgoing interrogation signal in amanner identical to the previous embodiments, except that a poweramplifier biasing element 505 and a power output detector 510 are addedto allow for precise digital control of output power. A temperaturemonitor 515 may also be included to prevent overheating. This embodimentalso includes a Tx video 501 input which is an analog signal generatedby a D/A converter. The Tx video 501 input is the analog equivalent ofthe Tx Symbol input referred to in FIG. 5. The Tx Mute 503 input allowsthe transmitter to be turned off and is independent of the receiver,i.e., the receiver may be listening to other transmissions while thetransmitter is shut-off. The ALC (automatic level control) 507 performsthe same function as the VGA 110 described with reference to theprevious embodiments.

In the present exemplary embodiment the incoming signal may again bepassed through a band-pass filter 145 and an AGC element 150. The signalmay then be demodulated in-phase and in-quadrature using demodulators155I and 155Q. The resulting base band signals may be passed throughlow-pass filters 160I and 160Q. The low pass filters 160I and 160Q areanti-aliasing filters for the D/A converters. This exemplary embodimentmay utilize the digital sub-system (shown in detail in FIG. 7) toperform other functions. For example, digital filtering of the Irx andQrx video signals 520I and 520Q in order to drive the feedback path andto enable real-time adaptation of the system depending on the multi-pathsignal propagation conditions.

The RF echo cancellation low pass filters may be digitally implementedin the baseband portion of the system. The multi-path signal propagationconditions change the nature of the echo signals from non-RFID elementsthat may be moving around in the environment. Thus, a digitallyimplemented adaptive filter may be advantageous. The inputs foradaptation may be a calibration period that sends out a known signalwhile obtaining reflections from known tags. For example, a known tagmay be affixed to a known location on the wall near a docking bayportal. The digital system may also continuously re-calibrate thefeedback loop by monitoring the video signals 520I and 520Q forimbalances. When such imbalances are detected the digital system maycompute gain, phase, and offset correction factors, and then apply thesefactors to the feedback loop using Icancel and Qcancel signals 525I and525Q.

The embodiment of FIG. 6 also shows a balanced amplifier 512 in thefeedback loop. The balanced amplifier may operate in the same manner asthe amplifier 175 described with reference to the previous embodiments.However, the balance amplifier 512 may be used for impedance matching tothe low pass filter 514. It should be understood that the low passfilter 514 is optional and does not need to be included in the feedbackloop of this embodiment. In the event that the low pass filter 514 isnot included, the balanced amplifier 512 may still be used to matchimpedance to the coupler 205 to reduce non-linearities.

FIG. 7 shows the sub-system interfaces through which the transceiverdescribed by the present invention may interact with other components ofan RFID system 1. A data conversion block 600 may provide a simpleanalog signal interface with digital controls. A programmable logicdevice 605 may provide a parallel interface to a digital signalprocessor 610. An MCU 615 may provide additional user controls andinterfaces with the transceiver and other components. The MCU 615 mayprovide the protocol for the communications between the reader and thetag including multi-tag arbitration. Such protocols may include thosepublished by the UCC (Uniform Code Council), EAN (European ArticleNumbering), and ISO (International Standards Organization). The MCU 615may also provide packet data synthesis for conversion to a bit streamthat may be bit encoded, modulated and transmitted by the transceiver,provide frequency hopping and channel access protocols, provideautomatic gain control for a maximum signal to noise ratio and dynamicrange of the received signal and provide automatic level control fortransmission power control for power savings, interference mitigation,user selectable power profiling, and any applications where powercontrol is advantageous.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the structure and themethodology of the present invention, without departing from the spiritor scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

1-19. (canceled)
 20. A receiver, comprising: a demodulator configured todemodulate a signal into an in-phase signal and a quadrature signal; anda base-band digital radio configured to receive the in-phase signal andthe quadrature signal and implement a feedback loop thereon, thefeedback loop configured to provide adaptation to cancel a portion ofradio frequency echo signals in the signal.
 21. The receiver of claim20, wherein the adaptation comprises real-time adaptation.
 22. Thereceiver of claim 20, wherein the base-band digital radio comprisesdigital components configured to digitally implement echo cancellation,and wherein the digital components are configured to adapt to cancel theportion of radio frequency echo signals in the signal.
 23. The receiverof claim 20, wherein the digital components are configured to adaptdepending on multi-path signal propagation conditions due to movement ofobjects and the receiver relative to one another.
 24. The receiver ofclaim 20, wherein the wherein the base-band digital radio is configuredto continuously re-calibrate the feedback loop by monitoring thein-phase signal and the quadrature signal for imbalances.
 25. Thereceiver of claim 24, wherein the wherein the base-band digital radio isconfigured to, upon detecting the imbalances, compute gain, phase, andoffset correction factors, and to apply these factors to the feedbackloop.
 26. The receiver of claim 22, wherein the digital components areconfigured to adapt for a calibration period by receiving a reflectionsignal from a known signal interrogating a known tag.
 27. The receiverof claim 22, further comprising: filters disposed between the in-phasesignal and the base-band digital radio and between the quadrature signaland the base-band digital radio.
 28. The receiver of claim 22, whereinthe feedback loop outputs an in-phase cancellation signal and aquadrature cancellation signal.
 29. The receiver of claim 28, furthercomprising: a modulator receiving the in-phase cancellation signal andthe quadrature cancellation signal and a combiner forming a feedbacksignal as a combination of the in-phase cancellation signal and thequadrature cancellation signal.
 30. The receiver of claim 29, furthercomprising: a coupler combining the signal and the feedback signal tocancel the portion of radio frequency echo signals in the signal. 31.The receiver of claim 30, further comprising: an amplifier disposedbetween the combiner and the coupler, the amplifier configured toprovide impedance matching.
 32. The receiver of claim 31, furthercomprising: a low pass filter disposed between the amplifier and thecoupler, the amplifier providing impedance matching to the low passfilter.
 33. A base-band digital radio, comprising: inputs for anin-phase signal and a quadrature signal; outputs for an in-phasecancellation signal and a quadrature cancellation signal; digitalcomponents configured to monitor the in-phase signal and the quadraturesignal in order to drive the in-phase cancellation signal and thequadrature cancellation signal; wherein the in-phase cancellation signaland the quadrature cancellation signal are configured to combine withthe in-phase signal and the quadrature signal to adaptively cancel aportion of radio frequency echo signals therein.
 34. The base-banddigital radio of claim 33, wherein the digital components are configuredto adapt in real-time to cancel the portion of radio frequency echosignals in a signal formed by the in-phase signal and the quadraturesignal.
 35. The base-band digital radio of claim 33, wherein the digitalcomponents are configured to adapt depending on multi-path signalpropagation conditions due to movement of objects and the base-banddigital radio relative to one another.
 36. The base-band digital radioof claim 33, wherein the digital components are configured tocontinuously re-calibrate the in-phase cancellation signal and thequadrature cancellation signal by monitoring the in-phase signal and thequadrature signal for imbalances.
 37. The base-band digital radio ofclaim 36, wherein the wherein the digital components are configured to,upon detecting the imbalances, compute gain, phase, and offsetcorrection factors, and to apply these factors to the in-phasecancellation signal and the quadrature cancellation signal.
 38. Amethod, comprising: demodulating an incoming signal into an in-phasesignal and a quadrature signal, the incoming signal comprising echosignals; digitally processing the in-phase signal and the quadraturesignal to provide real-time adaptation based on the echo signals;continuously re-calibrating an in-phase cancellation signal and aquadrature cancellation signal based on the processing; and coupling theincoming signal with a combination of the in-phase cancellation signaland the quadrature cancellation signal.
 39. The method of claim 38,wherein the echo signals comprising an error component based on any ofsignal energy coupled from a transmitter, signal energy reflected froman antenna, and signal energy reflected from non-radio frequencyidentification (RFID) elements in an environment.